The present invention relates to a color conversion filter substrate for displaying multiple colors with high definition, good environmental resistance, and high productivity, and to an organic multicolor emitting display device provided with such a filter substrate. More specifically, the present invention relates to a color conversion filter substrate and an organic multicolor emitting display device provided with such a filter substrate, especially an organic multicolor emitting display device using a color conversion method, for a display of electronic and electric equipment such as an image sensor, a personal computer, a word processor, a television, a fax machine, an audio equipment, a video equipment, a car navigation system, an electric desk top calculator, a telephone, a portable terminal, or an industrial instrument.
Since Tang et al. reported that a layered EL showed a high luminance of 1000 cd/m2 at an applied voltage of 10 V (Appl. Phys. Lett., 51, 913 (1987)), a great effort has been made to develop organic EL devices for a practical use. The organic EL device is a thin film self-emission element and provides a high current density at a low voltage. Therefore, it is expected to apply the organic EL device to a flat panel display.
Tohoku Pioneer Corporation has already developed products including vehicle-mounted green monochrome organic EL displays since November 1997. In order to meet the society needs, it is desirable to develop improved organic EL displays that can display multiple colors or full colors.
A method of displaying multiple or full colors with the organic EL display includes methods disclosed in Japanese Patent Publications No. 57-167487, No. 58-147989, and No. 03-214593, in which light emitting elements of the three primary colors (red, green, and blue) are arranged in a matrix form. In a case that the organic light emitting device is used to provide a multicolor display, it is necessary to arrange three types of light-emitting materials (R, G, and B) in a matrix form with high precision, thereby making it technically difficult to produce and increasing a cost. Further, the three types of light-emitting materials have different life times, thereby shifting a color of the display with time.
In Japanese Patent Publications No. 01-315988, No. 02273496, and No. 03-194895, a method in which a color filter and a backlight emitting white light are used to display the three primary colors through the filter has been disclosed. However, it is difficult to obtain an organic light emitting device emitting the white light with a long life, which is necessary to obtain bright three colors R, G, and B.
Japanese Patent Publication No. 03-152897 has disclosed another method in which phosphors arranged on a plane absorb light from light emitting devices, so that the phosphors emit fluorescence in multiple colors. Such a method using a certain luminous device to allow the phosphors to emit fluorescence in multiple colors has been applied to CRTs, plasma displays, and the like.
Further, in recent years, a color conversion method has been proposed in which a filter is composed of a fluorescent material for absorbing light with a wavelength in a light-emission region of an organic light emitting device, so that the fluorescent material emits fluorescence with a wavelength in a visible light region (Japanese Patent Publications No. 03-152897 and No. 05-258860). In this approach, an organic light emitting device that emits a color other than white can be used. Therefore, it is possible to use an organic light emitting device with higher brightness as a light source. In a color conversion method using an organic light emitting device emitting blue light (Japanese Patent Publications No. 03-152897, No. 08-286033, and No. 09-208944), a frequency of blue light is converted to that of green or red light. A color conversion filter containing a fluorescent material with such color conversion effect may be formed in a high resolution pattern. Accordingly, it is possible to provide a full-color light emitting display even with weak energy light such as near-ultraviolet light or visible light.
In order to form a pattern of the color conversion filter, a method in which a pattern is formed with a photolithography process after a film of a resist (photosensitive polymer) material containing fluorescent material is prepared by spin-coating has been disclosed in Japanese Patent Publications No. 05-198921 and No. 05-258860. Also, Japanese Patent Publication No. 09-208944 has disclosed a process in which a fluorescent material or fluorescent pigment is dispersed in a basic binder followed by etching the binder with an acid solution. The former method has been preferred because a manufacturing process is simple and the pattern formation exhibits good reproducibility.
For a practical application, it is necessary for a color display to possess long-term stability as well as high definition colors. However, in the organic EL devices using the color conversion filters, light-emission characteristics such as a current-luminance characteristic tend to degrade markedly after a specific period of time. A major cause of the degraded light-emission characteristics is degradation of the conversion capability of the color-converging filter associated with drive of the device.
A cause for the degraded conversion capability of the color conversion filter includes decomposition and quenching of fluorescent pigments caused by radicals. When a matrix resin is formed by using a resist, radicals are generated from photo-polymerization initiators and/or thermo-polymerization initiators (curing agent) contained in the resist, or growth radicals are generated from reactive mono-functional and/or multi-functional monomers and oligomers. As a result, it is known that the radicals can attack the filter to thereby decompose and/or quench the organic fluorescent pigment. In fact, the inventors confirmed that when the color conversion filter was formed in a pattern through position-selective curing of the resist into a matrix resin, the conversion capability was deteriorated due to light irradiation in an exposure step or thermal stress in a drying step.
Furthermore, after the color conversion filter is formed in a pattern, unreacted photo-polymerization initiators and/or thermo-polymerization initiators or unreacted reactive mono-functional and/or multi-functional monomers and oligomers may remain in the patterned film. In such a case, external light or light from the device can generate radicals from the unreacted substances, thereby degrading the conversion capability of the color conversion filter. That is, driving the device may degrade the conversion capability of the color conversion filter.
Another cause for the degraded conversion capability of the color conversion filter is a decrease in a fluorescent quantum yield resulting from a change in a dielectric constant of the matrix resin near the fluorescent pigment. It is generally known that a dielectric constant of a substance decreases as a cross-link reaction of the resist progresses. For example, methylmethacrylate (monomer) has a dielectric constant of 4.0. However, when it is polymerized and solidified, the resulting poly-methylmethacrylate has a reduced dielectric constant of 2.9.
That is, an environment of the fluorescent pigment is changed through an interaction between the organic fluorescent pigment molecules and the cross-linked matrix resin. As a result, the fluorescent quanta yield decreases. Also in this case, when unreacted photo-polymerization initiators and/or thermo-polymerization initiators or unreacted reactive mono-functional and/or multi-functional monomers and oligomers remain in the patterned film, driving the device causes the cross-link reaction of the resist, thereby degrading the conversion capability of the color conversion filter.
It is therefore an object of the present invention to provide a color conversion filter exhibiting such stable performance that the conversion capability remains unreacted in the long-term drive of the device.
Further objects and advantages of the invention will be apparent from the following description of the invention.